Almasi, B. Effects of Stress on the Barn Owl (Tyto alba) and the Link to Melanin-based Coloration. 2008, University of Zurich, Faculty of Science.

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Abstract

The present thesis investigates the effects of a short period of a chronically elevated corticosterone level, the main stress hormone in birds, in an ecological context and draws a link to the mechanisms explaining individual differences in the sensitivity to stress. The barn owl, as a model species, provides excellent opportunities to investigate parental investment and nestling development in natural conditions and allows experimental manipulations. I chose an experimental approach to manipulate stress hormone levels in free-living barn owls, using newly available self-degradable pellets releasing corticosterone. These pellets proved to be a powerful tool to elevate corticosterone levels. The hypothalamic-pituitary-adrenal (HPA)-axis responded strongly to the corticosterone administration resulting in a decreased HPA-axis responsiveness as demonstrated by the decreased endogenous response to an acute stressor of corticosterone-implanted birds (chapter 1). Furthermore, the increase of circulating corticosterone after implantation of the pellet and the regulation of free corticosterone through corticosterone-binding globulins (CBG) varied with environmental conditions and food regimes of the nestlings (chapter 6). These results imply that for an understanding of the biological relevance of effects of stress it is not sufficient to study laboratory animals or animals in captivity. On the contrary it is crucial to study free-living animals and observe the context the animal is living in; otherwise we most likely miss or misinterpret important aspects of effects of stress. Elevated corticosterone levels in breeding barn owl males resulted in a decreased male investment into the brood, which the female did not compensate for, but were not inhibitory to current reproduction (chapter 2). In nestling barn owls a short period of increased corticosterone levels caused a decrease in body mass gain, wing length, and tarsus growth. These reduced growth rates persisted much longer than the corticosterone levels were elevated; at fledging body mass and wing length of corticosterone-implanted nestlings were still lower than in untreated nest mates (chapter 3). Elevated corticosterone levels clearly entailed costs in terms of reduced resistance against oxidative stress, and, because antibody production was reduced, in terms of a higher risk of infections and progression of diseases (chapter 7). All these results together suggest that a short period (only two to three days!) of elevated corticosterone levels influence strongly parental investment and nestling growth and have far longer-lasting effects than the effects that are manifest during the time of elevated corticosterone levels. Such a short period of stress can ultimately shape the phenotype and most likely also influence fitness at adulthood. I used the hypothesis suggested by Ducrest et al. that eumelanin-based coloration is genetically associated with resistance to stress in vertebrates and formulated the prediction that darker, more eumelanic, individuals are more resistant to stress. This hypothesis is based on a literature review of genetic and pharmacological studies. Thus it is crucial to test experimentally whether the degree of eumelanin-based coloration is indeed associated with resistance to stress. In this thesis I performed such experiments in breeding males and nestlings. I found that elevated corticosterone levels affected darker individuals less than whiter ones. More eumelanic males reduced provisioning rates less than whiter males (chapter 2), nestling growth was less affected in darker nestlings (chapter 3), and nestlings of darker genetic mothers had lower total and free corticosterone levels after corticosterone administration than nestlings of whiter mothers (chapter 4). These results together support the hypothesis that the degree of eumelanin-based coloration signals the ability to cope with stressful environmental situations and may explain why melanin-based coloration is a mate choice criterion. We could also demonstrate that elevated corticosterone levels influenced feather phaeomelanin production, which suggests that corticosterone indeed influences the condition-dependent part of melanin production and could therefore reflect the fitness of the individual during feather growth (chapter 5). The present thesis adds new information on the signaling function of eumelanin-based coloration in the barn owl, as dark owls were significantly better able to cope with an experimental elevation in corticosterone levels than lightly colored owls. Stress sensitivity is an important trait since it influences many other fitness components and when signaled in melanin-based coloration can play a role in sexual selection.